The present invention involves a method and apparatus of registering or calibrating a tomosynthesis system. In particular, the present invention is directed to a rapid and accurate method of registering an energy source to a sensor surface in a tomosynthesis system.
In a number situations is useful to view the detailed internal structures of solid objects or to see inside of objects that are opaque to the visible spectrum. In medical applications, for example, it is desirable to non-invasively detect structural details of bone or tissue within a human body or other living organism. The diagnosis and treatment of disease and injury often depends upon such capabilities. Other applications include, for example, the nondestructive testing and inspection of material joints, electronic assemblies, and other intricate devices or high value articles. Similarly, it is also useful to inspect and examine objects and containers for the presence of contraband, listening devices, explosives or dangerous materials.
Several conventional techniques have been used to view internal structures with varying degrees of success. Most of these conventional systems use energy in the form of penetrating radiation, such as x-rays or gamma rays. Magnetic resonance imaging (MRI) systems can also be used to view internal information. The conventional systems, however, tend to be time consuming, expensive and have severe limitations in terms of the amount of visual information provided about detailed internal structures of objects such as soft tissues and the surface characteristics of internal organs of human bodies.
Most conventional systems involve the use of penetrating radiation directed through the object onto radiographic film or a bank of electronic radiation detectors. One such conventional technique, computed tomography (CT), involves a radiation source mounted in view of a row of radiation detectors. An object placed between the radiation source and the detectors is rotated to expose a section of the object to radiation from several angles. The data collected by the radiation detectors is processed to generate a two-dimensional slice of the internal structure of the object. To obtain a three-dimensional view, a number of slices of the object must be taken at discrete distances and, even then, information will often be lacking between slices. Hence, conventional CT techniques are not particularly well suited for the generation of three-dimensional views since the object must be exposed to additional radiation for each slice.
U.S. Pat. No. 4,167,672 to Richards pertains to another conventional technique known as dynamic tomography or tomosynthesis. In accordance with this technique, the object is exposed to radiation from a number of angles, with each exposure recorded on a separate piece of radiographic film. The set of radiographs may be viewed by superimposing and appropriately aligning them to bring an image plane into focus. This tomosynthesis technique avoids the overexposure problem of the single slice CT systems, since the object is subjected to a fewer number exposures to obtain three-dimensional information for the object. Obtaining three-dimensional information with tomosynthesis can be quite difficult, however, due to problems such as film and equipment misalignment and inaccuracies. Also, the task of adjusting or manipulating a stack of radiographs to obtain a clear three-dimensional image or a desired image can be enormously complex. Also, images can only be viewed in planes which are parallel to one of the film planes in the set of radiographs.
FIG. 1 depicts a portion of the tomosynthesis system 100 disclosed in U.S. Pat. No. 5,070,454 to the present inventor. In the system of FIG. 1, an object 10 is placed on platform assembly 20 for examination. The platform assembly comprises rotation plate 30 made of a material that is substantially transparent to penetrating radiation, and a film cassette 40 for holding radiographic film sheets 80. The rotation plate 30 may be rotated so as to expose the object 10 to radiation at a number of different rotational positions relative to radiation source 50. The ""454 patent explains the invention in terms of having eight different rotational positions, but discloses that different numbers of rotational positions can be used in accordance with the invention. Two circular reference markers 60 and 70 made of a high-density radiation opaque substance are mounted in the platform assembly 20 between the rotation plate 30 and the film cassette 40. In accordance with one embodiment of the ""454 patent, the reference markers 60 and 70 are positioned to form an equilateral triangle between their centers at the base of the triangle and the center of the rotation plate 30 at the apex of the triangle.
To collect images of the object 10 for tomosynthesis, a new radiographic film sheet 80 is used at each rotation position to capture an exposure from the radiation source 50. That is, each sheet of radiographic film 80 records a radiographic image of the object 10 at one of the eight distinct angular orientations. A video camera 90 can be used to digitize the images of the developed radiographic film sheets 80 containing the radiation shadows of the object 10 and the reference markers 60 and 70. The digitized images may then be supplied to a computer 101 for further processing.
In accordance with the novel tomosynthesis system disclosed in the ""454 patent, the shadow images of the object 10 must be properly aligned to bring the tomosynthesis image into focus and view the planes at various depths of the object 10. The dual reference markers 60 and 70 are used to properly aligned the shadow images and thus register the shadow images of the radiographic film sheets 80 which have radiation shadows of the object 10 from various angles. This allows an operator of the tomosynthesis apparatus to select a level or series of levels desired for viewing selected internal structural images of the object 10.
Embodiments of the ""454 patent have the two registration marks of the reference markers 60 and 70 on each of the exposed radiographic film sheets 80 containing shadow images from a different position. The two registration marks facilitate appropriate registration of align the exposed radiographic film sheets 80 for tomosynthesis and can be used to address matters such as errors due to equipment orientation and misorientation. One drawback to the use of the two registration marks on the rotatable platform is that the marks occupy space on the platform, as well as on the developed radiographic film, that could otherwise be used to support an object to be studied. Because of the expense of film and the increased difficulty in accurately registering equipment as the size of the equipment gets larger, it would be desirable to limit the amount of space in a tomosynthesis system that must be dedicated to determining rotational position and ensuring appropriate equipment registration.
Also, in accordance with the present invention, the inventor realized that the two planar reference markers 60 and 70 do not always provide sufficient information about the orientation of the radiation source 50 relative to the platform assembly 20. By having the two registration marks on each radiographic film sheet 80, such relative orientation information was not necessary. Such information is useful in embodiments of the present invention using a plate sensor instead of radiographic film sheets. Particularly for tomosynthesis using a plate sensor or sensor assembly instead of radiographic film sheets, it is advantageous to know the precise alignment and relative orientation of the energy source and the sensor.
The present invention permits rapidly and accurately registering a sensor source to a sensor surface in a tomosynthesis system.
In accordance with one aspect of the present invention, a method of determining an orientation of an energy source relative to an energy sensor having a sensor surface is disclosed. According to the method, a first registration marker is placed on a marker location proximate the sensor surface. A first shadow image corresponding to the first registration marker is obtained with the sensor surface by exposing the first registration marker to energy from the energy source. A second registration marker is placed at a predetermined height above the marker location on the surface. A second shadow image corresponding to the second registration marker is obtained with the sensor surface by exposing the second registration marker to energy from the energy source. A location of the first shadow image and a location of the second shadow image are compared to determine an orientation between the energy source and the sensor surface.
In accordance with another aspect of the present invention, a method of obtaining an object image in a tomosynthesis system, the system including an energy source and an energy sensor having a sensor surface, is disclosed. According to the method, an orientation of the energy source relative to the sensor is determined. An object is positioned proximate the sensor surface. One or more object shadow images are obtained with the energy sensor by exposing the object to energy from the energy source. The one or more object shadow images are manipulated as a function of the orientation between the energy source and the sensor surface.
In accordance with yet another aspect of the present invention, a tomosynthesis system including a registration arrangement includes an energy source, and a sensor adapted detect energy from the energy source and having a sensor surface. The system further includes a first registration marker adapted to be removably disposed at a marker location proximate the sensor surface, and a second registration marker adapted to be removably disposed at a predetermined height above the marker location. A processor is provided for comparing locations of shadow images of the first registration marker and the second registration marker obtained by the sensor by exposing the first registration marker and the second registration marker to energy from the energy source to determine an orientation of the energy source relative to the sensor surface.
In accordance with still another aspect of the present invention, an apparatus for representing an internal structure of an object includes an energy source, an energy sensor for sensing energy from the energy source and generating image data pertaining to an object disposed proximate the energy sensor and exposed to energy from the energy source, and a processor programmed to manipulate image data obtained from the energy source as a function of an actual orientation between the radiation source and the energy sensor from which an image of the object can be generated.